The present invention relates to water soluble dithiocarbamate salt polymers and their use in a metal scavenging process. More particularly, the present invention provides a method for removing metals from a fluid stream to below detection limits with the use of a polymeric dithiocarbamate.
As more problems are caused by contamination of rivers, waterways and the like by waste water from industrial sources, stricter regulations have been enacted to avoid contamination. For instance, a mandatory requirement exists to control metals in waste water below their prescribed concentrations. Particularly stringent regulations have been established for metals harmful to humans, such as mercury, cadmium, zinc, copper, lead, nickel and silver. For example, regulatory agencies have introduced stringent lead standards, resulting in a demand for new treatments that are able to remove lead to extremely low levels (&lt;0.1 ppm in many cases).
Various processes have been proposed to remove metal ions in waste water. In one process, metal removal is achieved using simple sodium salts of dialkyl dithiocarbamates (e.g. sodium dimethyldithiocarbamate, DMDTC). Several drawbacks exist in using low molecular weight dithiocarbamates. The precipitate formed can be small and settling or filtering of the precipitate from the waste water can be slow.
Moreover, current chemical treatments, such as diakyldithiocarbamates, require the separate addition of flocculants and/or coagulants for subsequent separation of metal-bearing precipitate from effluent water. In treating dilute rinse waters having low concentrations of metals, say below 50 ppm, the resulting metal-DTC complexes form colloidal particles in suspension; i.e., particles that will not settle in a reasonable time period. In some waste waters, these particles are so small that they appear only as color, carrying metal ions into the effluent. Thus, sufficient removal of the metal ions from the plant effluent necessitates the practice of appropriate coagulation/flocculation techniques for colloidal suspensions.
This coagulation/flocculation step is extremely difficult to optimize. Several characteristics of waste water systems, such as waste waters from metal plating operations in metal fabricating shops and printed wiring board manufacturing, makes the aggregation of metal-DTC particles especially problematic. Numerous dispersants are present from periodic sewering of chemicals such as cleaners, dissolved, photo-resistive masking agents, spent developer, soluble oils, and solder strip. These dispersants inhibit the aggregation of particles, increasing the dosage requirement for coagulant and sometimes, flocculent. Due to the changing concentration of these dispersants over time, coagulant dosage requires frequent adjustments, which are not made often enough to optimize the coagulation/flocculation step.
Additionally, the coagulation/flocculation step often precludes the ability to achieve low limits for metal discharge because of inadequate separation of the metal-bearing precipitate. In many cases, when the initial metal concentrations are low, say below 5 ppm, the current DTC chemicals are unable to polish the metal levels to below effluent discharge limits, typically at 50-500 ppb.
One proposal for improving the ability of dithiocarbamates to remove metals is to react carbon disulfide with polyethyleneimine polymers. Such a proposal is set forth in European Patent Application 0 090 551 A1. The polyethyleneimine dithiocarbamate polymers described in this patent are water insoluble. While evidencing the ability to chelate undesirable metals, the separation of the chelated metal polymer complex is relatively slow. The action of these materials is believed to be similar to the action of water insoluble ion exchange resins.
Another example of polyethyleneimine-dithiocarbamate polymers is set forth in U.S. Pat. No. 4,670,160. In Example 8 of the '160 patent, a polyethyleneimine resin having molecular weight of 60,000 was modified with carbon disulfide. The finished resin, while evidencing some water solubility, contained substantial quantities of insoluble materials.
Still another problem with prior processes has been the inability to readily collect the resulting precipitate. In some waste water systems, flow rates are so rapid that the precipitate within the waste water is not able to settle. In addition, some systems have no retention time and no site within the system in which the precipitate may settle. Still further, some solids in the water system are so small in diameter that they are nonsettleable.
As will be shown hereafter, the polymeric dithiocarbamate polymers of this invention are water soluble, have the ability to readily complex undesirable metals that are easily removable from aqueous solutions. Moreover, the method of the present invention assures the collection of all formed metal complexes, regardless of the flow rate.